Technical Field
Embodiments of the subject matter disclosed herein relate to methods, apparatuses, and systems for processing seismic data.
Discussion of the Background
Recently, interest in developing new oil and gas production fields has dramatically increased. However, the availability of land based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys in order to more accurately decide where or where not to drill (to avoid a dry well).
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, improving the resolution of images of the structures under the seafloor is an ongoing process.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows an array of seismic receivers 12 provided on cables 14 that form streamers 16 below a surface 18. The streamers may be disposed horizontally, arranged in a slanted or curved configuration, or be placed at multiple depths. The vessel 10 also tows a sound source assembly 20 that is configured to generate an acoustic wave 22a. The acoustic wave 22a propagates downwards toward the seafloor 24 and penetrates the seafloor until eventually a reflecting structure 26 (e.g., reflector R) reflects the acoustic wave. The reflected acoustic wave 22b propagates upwardly until it is detected by receiver 12 both directly and via a ghost reflection wave 22c. The recorded data is then processed for producing an accurate image of the subsurface. The processing may include various phases, e.g., velocity model determination, pre-stack, migration, post-stack, etc., which are known in the art and thus, their description is omitted herein.
A promising method for seismic data processing is reverse-time migration (RTM). Reverse-time migration uses a two-way wave equation in depth migration and has shown that in complex subsalt and salt flank areas, it is easier to incorporate amplitude corrections than the traditional methods. RTM often correlates the source seismic data wavefield (Ds) and the receiver seismic data wavefield (Dr) as a function of a time shift value (τ) by correlating forward propagated source seismic data (Ds(x,t∓τ)) with reverse propagated receiver seismic data (Dr(x,t±τ)). The correlation may be conducted for various propagation time shift values (τ) in order to provide time shift correlation data (I(x,τ)) as a function of the time shift (τ) for the selected position (x). In certain embodiments, the correlation is conducted according to the equation:I(x,τ)=∫Dr(x,t∓τ)·Ds(x,t±τ)dt  (1)
A disadvantage of current RTM processing techniques is that various artifacts are generated such as those shown in FIG. 2. For example, low frequency noise may generate one or more low frequency artifacts 210 and shear-like noise may generate one or more shear-like noise artifacts 220.
Given the foregoing, a need exists for improved migration processing, including RTM processing, that substantially eliminates artifacts such as those mentioned above.